1. Field of the Invention
Inevitably, railroad tracks and vehicular traffic roads have to cross each other at some locations. It is conventional to provide warning devices at such intersections in order to provide a signal to motorists indicating that a train is about to cross the intersection. The warning device has traditionally taken any of a rather wide variety of forms and may include one or more of flashing lights, audible alarms, and barriers. It has long been the practice to actuate such crossing alarm devices in automatic response to the presence of a train. Typically, a track circuit detects the entry of a train within the critical area and the crossing alarm device is actuated. However, this can cause an unnecessary actuation of the crossing alarm device if the train enters the area of the track circuit, but does not cross the road. This condition may occur when a train approaches an intersection, stops and/or reverses. This may occur as a result of a switching operation or any number of other circumstances with which those familiar with the railroading art are acquainted. A wide variety of sophisticated controls have been developed in order to avoid prolonged and unnecessary actuation of the crossing alarm device when a train is not crossing the road.
Devices known as motion detecting units, hereinafter sometimes identified as MDU, or DMU, have been designed which will detect approaching train motion and cause the crossing alarm device to be actuated only when there is actual train motion. While an MDU provides increased convenience in the actuation of the crossing alarm device, they are not always used due to the cost thereof. Consider for example a situation wherein several parallel tracks cross a highway. It would be necessary to provide an MDU for each set of tracks. Especially in applications wherein the road is a secondary road and/or the number of trains crossing per day is small, it has been considered too expensive to provide an MDU on each track. In such situations, alternate signal control methods may be used which, although more economical, sometimes result in actuating the alarm signal unnecessarily.